<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>

    <input type="text" id="user">
    <span id="msg"></span>

    <script>

        var oUser = document.getElementById('user');
        var oMsg = document.getElementById('msg');


        // 一边输入一边验证
        oUser.oninput = function () {
            oMsg.innerHTML = oUser.value;
        }

    </script>

</body>

</html>